1. Field of the Invention
The present invention relates to bipolar emitting fluorescent bis-coumarin compounds suitable for use in organic light-emitting devices (OLEDs). The invention further relates to a method of making such bis-coumarin compounds and to OLEDs employing such bis-coumarin compounds in an active layer.
2. Description of the Related Art
Coumarins are a class of well-known fluorescent compounds. They are widely used as laser dyes which can be incorporated into a variety of OLED structures and materials. Many commercially available coumarins typically contain an alkyl-substituted amino group on the phenyl ring. Although these coumarins have good photoluminescent yield, they have poor thermal stability, in that they exhibit thermal degradation even at temperatures below 400xc2x0 C.
Certain bis-coumarin compounds per se are known in the art. U.S. Pat. No. 4,240,967, for example, describes a class of pigments comprising two coumarin groups linked via a biphenyl or substituted biphenyl diamine bridge; the hetero-ring serves as the attachment site for each of the coumarin groups. These bis-coumarins are not described as thermally stable and likewise are not disclosed as light-emitting materials for use in OLEDs.
Triarylamines are well-known organic hole transport materials used in OLEDs, especially N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di-m-tolyl-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (TPD), N,Nxe2x80x2-di-1-naphthyl-N,Nxe2x80x2-diphenyl-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (NPD), and their analogs.
The use of coumarins and triarylamines in OLEDs is known in the art. For example, U.S. Pat. Nos. 5,126,214, 5,693,428, 5,763,110, 6,020,078 and 6,172,264 each discloses the possible use of a conventional coumarin as an emitting material, as well as the use of a triarylamine as a hole transport material, in a given OLED. However, as indicated above, commercially available coumarins typically suffer from thermal instability. As a result, OLEDs incorporating such commercially available coumarins will exhibit reduced quantum and power efficiencies, as well as reduced life-times.
It is an object of the present invention to provide fluorescent emitting materials suitable for use in OLEDs and exhibiting excellent thermal stability and good photoluminescence.
Accordingly, in one aspect the invention is a fluorescent compound having a structure 
wherein R is substituted or unsubstituted alkyl; wherein Ar2 is optionally present; and wherein Ar1, Ar2, and Ar3 are independently each a substituted or unsubstituted aryl group.
In further aspects the present invention is an organic light emitting device comprising a transparent electrode; a cathode; and an active layer containing a fluorescent compound having the structure 
In a related aspect, the invention is a method of making a fluorescent compound having a structure 
comprising the step of reacting an arylamino-substituted coumarin and a halo-substituted aryl in the presence of a catalytic metal.